Various conventional lidar systems (i.e., laser radar systems) employ coherent detection, in which a received optical signal is combined with a mixing or reference optical signal, typically with a symmetric combining coupler, to produce an interference signal. A symmetric combining coupler is a combining coupler with a power transfer ratio of 50% between its two output ports (i.e., each port receives 50% of the output power from the combing coupler).
In some conventional systems, the interference signal from a single port of the symmetric combining coupler is applied to a detector to produce a detected signal. This is referred to as single-ended detection. Because the two ports of the symmetric combining coupler each produce an interference signal with a 180-degree phase difference from one another, these two interference signals may be combined to produce a differential signal that can subsequently be applied to a detector to produce a detected signal. This is referred to as differential detection.
Optical systems employing single-ended detection typically employ fewer detectors and require fewer optical fiber splices. However, such optical systems employing single-ended detection experience a 3 dB loss in signal-to-noise ratio (SNR) over those employing differential detection. Because of this, optical systems that require higher sensitivity (such as lidar systems) typically employ differential detection with symmetric combining couplers rather than single-ended detection.
What is needed is a lidar system that employs single-ended detection that does not suffer the SNR losses of conventional systems.